Virtual Network Peering

Vnet peering enable communication between vnets, with in the same region, different regions, different subscriptions and ofcourse different Tenants. 

Vnets will helps with isolation which means resources accross vents will not communicate with each other, hence the need of Peering. 

Peering between the Vnets in the same Region is called Regional Vnet Peering. 

Peering between the Vnets in the different Regions is called Global Vnet Peering.

Vnet Peering traffic will go through private network of azure, which give you below advantages.
    *Secure
    *Reliability
    *Better performance

Peering should always two ways, If you create onway peering, two way communication will not work.

If you want allow traffic one way the other way to be bloked do it by Network Security Groups (NSG).

Here is the video which help you with the configuration of both Regional and Global Vnet Peering.